This invention generally relates to power converter systems and more particularly to a power converter system incorporating a full-wave bridge circuit comprised of thyristors connected in all of bridge arms.
In order to feed electric power from an arc power source to a dc load, a power converter system is generally used incorporating a full-wave bridge circuit. Since the dc output voltage delivered from the power converter system is varied by controlling the firing angles of thyristors comprised of the full-wave circuit, the phase of ac current lags with respect to ac voltage, giving rise to the generation of reactive power and the degradation of power factor.
In a most popular method of controlling the firing of thyristors, the firing angles of groups of thyristors comprised of a full-wave bridge circuit on both the positive and negative sides thereof are made equal to each other and the thyristors are fired sequentially and the dc output voltage is controlled by varying these firing angles. With this method, however, as the dc output voltage becomes low, the phase of ac current lags with respect to ac voltage correspondingly, resulting in increase in reactive power and decrease in power factor. These defects are of course causes for increased power consumption and additionally for an increased capacity of the full-wave bridge circuit which in turn adversely affects the power source.
Apart from the aforementioned method for firing controlling, the firing controlling method is known wherein the firing angles of the groups of thyristors on both the positive and negative sides are made different from each other so that a period may occur through which thyristors on both the positive and negative sides, connected in series, are fired simultaneously. With this method, however, when the dc output voltage is high, the form factor is degraded and additionally, the ac power source generates higher harmonics. Accordingly, if the resulting dc power is supplied to a dc motor, for example, the dc motor will pulsate and generate acoustic noises. The higher harmonics, especially, second harmonics associated with the power source raises a problem of inductive interference as well as yields a cause for erroneous operation of relays possibly connected on the power source side. This firing controlling method, therefore, cannot be applicable to a power converter system having a large capacity.
As described above, many problems were involved in the conventional power converter systems and the advent of an excellent power converter system to solve these problems has been desired.